The invention relates to power units including an internal combustion engine, and more particularly a Diesel engine having a low compression ratio, supercharged by a supercharging turbocompressor unit comprising a turbine which, when the engine is in operation, receives the exhaust gases thereof, the compressor outlet being connected to the turbine inlet by bypass means disposed parallel to the engine, permanently open in operation and arranged to pass that portion of the compressed air delivered by the compressor which is not absorbed by the engine with a head loss which, if appreciable, is substantially independent of the ratio of the flow rate traversing said bypass means to the air flow delivered by the compressor, and an auxiliary combustion chamber having an adjustable fuel supply being disposed upstream of the turbine.
In internal combustion engines of the afore-mentioned type, the provision of the bypass means renders it possible to operate the turbocompressor near its surge line, i.e. with very high efficiency, which is particularly important for Diesel engines operating with a high supercharging pressure and a low compression ratio (typically lower than 12).
Starting and low-load operation of engines of the aforementioned type presents problems when the ambient temperature is low. These problems are particularly acute with Diesel engines having a low compression ratio (e.g. below 12).
The difficulties resulting from very low external temperatures can be overcome by heating the air fed to the engine, for instance by increasing the compression of the air supplied by the compressor, which involves increasing the amount of fuel injected into the combustion chamber disposed upstream of the turbocompressor turbine. This method, however, increases the fuel consumption.
By way of example, in the case of a 3000 hp engine having a compression or volumetric ratio of 7, starting and idling are easy if the temperature of the air supplied to the engine is 110.degree. C. If the ambient temperature is -30.degree. C, a compression ratio of 3.5 is required for heating to 110.degree. C by adiabatic compression (efficiency 0.75) of the air in the compressor. This ratio can be obtained if fuel is introduced into the combustion chamber at a rate of 140 liters per hour.
The operating parameter determining good starting and proper idling is the time delay before the air-fuel mixture ignites in the engine combustion chambers at the end of the compression stroke; this time depends mainly on the air temperature and, to a lesser extent, on the pressure. For example, the same time delay before ignition can be obtained with 3.5 bars and 110.degree. C or with 1.7 bars and 150.degree. C.
If, however, the ambient temperature is -30.degree. C and if the air is heated only by adiabatic compression in the compressor, the temperature of air delivered at 1.7 bars will be 23.degree. C only. If, furthermore, the head loss between the compressor and the turbine is 15% of the pressure delivered by the compressor, the temperature of the hot gases supplying the turbine will have to be 402.degree. C for the turbocompressor to be able to operate independently.
The air fed to the Diesel engine can be economically reheated by recycling part of the gas leaving the combustion chamber towards the engine intake. The recycled gas is mixed with the air supplied by the compressor at, e.g., 23.degree. C. Under the above conditions, it is sufficient to recycle 45% of the mass flow of hot gas at 402.degree. C into the air supplied by the compressor and to introduce fuel at the rate of 40 1/hr into the combustion chamber. The fuel saving is due to the fact that the heat in the flow of recycled hot gas is completely recovered. The bulk of flowing gas acts like a reserve storage of heat flowing in the bypass and only raises the temperature of the overall gas body in which engine intake and exhaust occur.
This saving is particularly important for engines which have to operate for long periods under idling or low-load conditions, e.g., marine engines driving a constant-pitch propeller which absorbs 12% of the maximum power at half the maximum speed and about 2% of maximum power at one-quarter the maximum speed.
Heating of the compressed air delivered to an engine by recycling hot gases has been used hitherto for a long time. However, recycling presents substantial additional problems when the compressor outlet is connected to the turbine inlet by continuously open bypass means disposed in parallel relation with the engine, particularly if throttle means are disposed in the bypass as disclosed in copending U.S. application Ser. No. 437,748, filed Jan. 29, 1974, in the name of Jean F. Melchior, applicant herein. Such throttle means and the resulting pressure drop are for satisfactory scavenging of the engine cylinders, which is essential in the case of a two-stroke engine.
It is known to recycle the exhaust gas by connecting the exhaust of the turbocompressor turbine to the compressor intake by a pipe (U.S. Pat. No. 2,633,698). However, large volumetric rates of flow are involved and the engine assembly should have a special suction and delivery-pipe installation.
U.S. Pat. No. 3,149,454 describes an engine which may operate either as a Diesel engine or as a gas engine, which is supercharged by a turbocompressor unit and which is devoid of any bypass conduit and auxiliary combustion chamber. A venturi nozzle is located between the output of the compressor and the intake of the engine for recycling part of the engine exhaust gas to the intake when the engine operates idle on gaseous fuel. Such recycling, if retained for Diesel engine operation, is entirely unsatisfactory. If an attempt is made to accelerate the idle engine, the fuel delivery to the engine is abruptly increased. That fuel is not completely burnt and is partly found in the partly recycled exhaust gas. White polluting smoke appears at the output of the exhaust pipe. The recycled exhaust gas still increases the richness in the engine combustion chamber and only renders combustion less satisfactory. It is well known that the time delay for ignition increases if the richness is excessive: if in spite of the poor combustion efficiency the speed of the engine tends to increase, the time delay for ignition corresponds to a crank angle which increases and may become greater than the angle at which the exhaust valves open. As a result, it would generally not be possible to increase the speed from idle under such conditions.
Still, since the pressure at the exhaust of an internal combustion engine oscillates and exhibits a peak at the beginning of the exhaust discharge, recycling can only be pulsed and consequently unsteady.